ctx.js

function ctx_to_object_url(ctx) {

	var img_bytes = atob(ctx.canvas.toDataURL().substring('data:image/png;base64,'.length)),
       	    ua = new Uint8Array(new ArrayBuffer(img_bytes.length));

        for (var i = 0; i <= ua.length; i++) {
            ua[i] = img_bytes.charCodeAt(i);
        }

        return (window.URL || window.webkitURL).createObjectURL(new Blob([ua], { type: 'image/png' }));

}